There are several ways to consider it. But the way I look at it, for the way I vape- I have to rebuild because the coil fouls and needs to be replaced or cleaned...before the wick reaches end of useful life anyway. So no matter what, both last as long as they need to.
Rayon has better heat resistance, but unless something goes wrong, that won't come into play. Rayon wicks more quickly & efficiently, because it has more linear, smoother fibers. But that has nothing to do with life, really. Because of of the structure of cotton, it kind of "filters" your juice more than rayon- which theoretically could actually prevent some of the tds or impurities from ever reaching the coil, and maybe keeping it cleaner longer.
Full disclosure: If all things were equal, I'd use rayon. But using mostly nets, I prefer the less "sharp" taste of cotton, and kgd is easier to work with and simply cut to size, rather than pulling apart the fibers of rayon, trying to be consistent. Just my $0.02. This could be vastly different for those that use unflavored/different flavors, or high wattages, or have other issues that would require wicks to be replaced before dry-firing or replacing the coil.